The field of invention relates generally to the field of printing. More specifically, the invention relates to test patterns used for adjusting spot spacings in multiple beam printheads.
Laser printing involves directing a beam of laser light to a photoconducting drum (also referred to simply as a drum). In a typical process, the laser illuminated drum regions electrostatically attract toner particles which are subsequently transferred to a piece of paper. Thus the laser illuminated drum regions typically correspond to the printed matter on the paper.
Multiple beam printhead technology involves simultaneously directing multiple laser beams onto the drum. Currently technology employs four or five laser beams per printhead. FIG. 1 shows a depiction of the operation a four beam printhead technology. FIG. 1 is not drawn to scale.
Referring to FIG. 1a, each laser beam has an associated spot location 101a-104a (also referred to as spots 101a-104a). The size and placement of spots 101a-104a are determined by the optical system of the printhead. The printhead also has the ability to sweep the spots 101a-104a from left to right across the drum surface 100. As FIG. 1a shows, the laser beams associated with spots 101a and 103a are xe2x80x9conxe2x80x9d while the laser beams associatedwith spots 102a and 104a are xe2x80x9coffxe2x80x9d. The spots spacing between spots 101a-104a is constant.
As the spots 101a-104a are swept across the drum surface 100, the regions 105a, 106a of the drum exposed to the xe2x80x9conxe2x80x9d laser beam are electrostatically altered such that toner materials are attracted to them. FIG. 1 therefore also shows the relationship between the spots 101a-104a and the printed end product.
FIG. 1b shows the printed matter regions 105b, 106b if the printhead continues to sweep the spot locations 101b-104b from left to right with the same coordination of xe2x80x9conxe2x80x9d and xe2x80x9coffxe2x80x9d laser beams as in FIG. 1a. That is, beams associated 101a, 103a of FIG. 1a and 101b, 103b of FIG. 1b remain xe2x80x9conxe2x80x9d while beams associated with 102a, 104a of FIG. 1a and 102b, 104b of FIG. 1b remain xe2x80x9coffxe2x80x9d.
FIG. 1c shows the printed matter regions 105c, 106c if the laser beam associated with spot location 103c is eventually turned xe2x80x9coffxe2x80x9d at some point after the situation shown with respect to FIG. 1b and the printhead continues to sweep the spots 101c-104c. FIG. 1d shows the printed matter regions 105d, 106d if the laser beam associated with spot 101d is turned off such that edges 108 of the printed matter regions 105d, 106d xe2x80x9cline upxe2x80x9d. FIG. 1d also shows the beam associated with spot 102d has turned xe2x80x9conxe2x80x9d at some later time. Thus regions 105d, 106d may also be viewed as the printed matter on the page once the printing process is complete.
Ultimately the sweep reaches near the right edge of the drum (which corresponds to the right edge of the finished page). The drum rotates up 4.0 PELS (which corresponds to lowering the spots to a new row just beneath the completed row) and the spots are swept again. Depending on the printing apparatus used, the spots may be swept from right to left during the sweeping of the second row; or, similar to a typewriter, the laser beam writing process may always start from the left. Other printers may always start from the right.
FIG. 2 shows a more detailed view of the printhead""s spacing of the spots 201-204. FIG. 2 shows a slanted angle printhead (also shown in FIG. 1). 1.0 picture element (PEL) is the xe2x80x9cdesigned forxe2x80x9ddistance 221a, b, c between spot center locations 210-213 (also referred to as spot location origins 210-213). The actual spacing between adjacent spot center locations 210-213 changes from printer type to printer type. In typical applications, the vertical distance 221a, b, c between the spot location origins 210-213 is designed to be 1.0 PEL while the horizontal distance 222a, b, c is designed to be 40.0 PELs. Printer addressability is typically measured in Dots Per Inch (DPI). A PEL is therefore 1/DPI in the vertical direction. Horizontal distance is the distance along the sweep direction, while vertical distance is the distance perpendicular to the sweep direction.
The quality of images produced by multiple beam printheads is sensitive to the vertical spacings 221a, b, c (also referred to as spacings). For example, if the spacings 221a, b, c are different than the spacings the image was supposed to be formed with (i.e., something other than 1/DPI of the designed for image), undesirable xe2x80x9cbandsxe2x80x9d (also referred to as banding) may appear in the image. Thus vertical spacings 221a, b, c are important to the quality of the printed image. The spacings 221a,b,c may be accurately set in a manufacturing environment with sophisticated equipment such as cameras or other alignment tools. However, those involved in field service repairs typically do not have access to such equipment. As such, a test pattern that assists field service personnel in accurate alignment without the use of alignment equipment would improve the optical alignment of field installed or field adjusted printheads.
A method is described comprising sweeping multiple beams for a plurality of sweeps. The multiple beams are modulated for the plurality of sweeps according to a test pattern. The test pattern has an associated vernier period and the vernier period corresponds to a wavelength. The wavelength is perceptible to the human eye.
An apparatus is described comprising a printhead configured to produce multiple beams where the multiple beams are configured to be modulated according to a stored test pattern. The test pattern has an associated vernier period and the vernier period corresponds to a wavelength. The wavelength is perceptible to the human eye.